The Milky Way is our home galaxy. A peculiar feature of this galaxy is a bright band of light that runs almost in a perfect circle through it.
As seen from the earth this band looks like a river of Sight following through the sky. Actually, it is made up of millions of scintillating stars which from this distance seems to be placed in close proximity to one another.
Legend has it, that in response to the prayers of a devotee Bhagiaratha, God Shiva brought the Akash Ganga down and allowed a trickle of it to fall on the Earth. This trickle formed the earthly Ganga (River Ganges).
The Milky Way is a spiral galaxy. The main body of the galaxy is a disc 100,000 thousand light years across with a globular nucleus of about 16,000 light years in diameter, and far-stretching spiral arms (in one of which solar system is located).
The galaxy consists of over a hundred billion stars rotating about the centre in a stately average period of about 230 million years.
The farther from the plane of the Milky Way, the fewer faint stars are visible in these directions the smaller is the distance to which the stellar system extends.
The solar system lies not in the centre of the Galaxy, which is visible from Earth in the direction of Sagitarius.
The stellar population of the Milky Way is made up of three categories of Stars:
(i) there are the stars in the hub and spirals of the Milky Way. The Sun belongs to this group of stars. These groups are called open or Galgatic Clusters.
(ii) Beyond the disc and the open clusters, lie the Halo stars. Many of these stars form miniature galaxies, containing tens of thousands of stars. These are called Globular Clusters. They contain very old stars.
(iii) Beyond the Globular Clusters there are several million individual stars, that run round rakishly on the outskirts of the Milky Way. All these form part of the galaxy.
One of the nearest Globular Cluster, the one in Hercules, is separated from us by 20,000 light years, its diameter being a hundred light years. The most distant star cluster lie 230,000 light years away from us.
The stars in the Galaxy are being continually reshuffled. In the course of a million years, two stars now close together but having a difference in velocity of one kilometre per second, will have separated by 3 light years.
Therefore, in less than one cosmic year (250 million years), some groups of stars may disintegrate and new groups of stars may form. The physical make-up of the galaxy is also likely to change over intervals of the order of a cosmic year.